The present invention concerns a procedure for measuring contents of hydrocarbons in liquids containing such, in which liquids the main component is water and which may in addition to hydrocarbons contain salts (seawater) or other dissolved chemicals (industrial emission waters). In the procedure, the liquid to be measured is conducted into a transparent measuring cuvette which is irradiated with IR radiation from a radiation source, and the contact of the hydrocarbon present in the liquid is determined on the basis of the attenuation caused by the hydrocarbon in the 3.4 to 3.5 .mu.m wavelength range.
The IR absorption method is commonly used, for instance, in laboratory determinations of the oil content of waters. An advantage of the procedure is its universal applicability based on the fact that the absorptions of different oil types at 3.4 .mu.m wavelength are very close to one another. Because of the strong absorption of water, it is however necessary, in laboratory measurements, to concentrate the sample before it is measured, this being accomplished by extracting the oil in carbon tetrachloride and separating from the water the extract thus obtained. As a consequence, the procedure is slow and introduces a risk of toxic emissions.
In a procedure known in the prior art, the absorption of water is compensated for by forming, from the sample proper, a reference sample from which the oil has been removed. When these two are pumped through the cuvette in alternation, the attenuation due to the oil can be measured. In order to achieve sufficient measuring accuracy, the differential temperature between the samples is equalized prior to the cuvette with the aid of a heat exchanger, and for eliminating the differential pressure, they are always stopped in the cuvette for the duration of measurement. To produce the reference sample, an ultrafilter is employed, which removes, on the side of the oil, any solid particles which may occur therein. The attenuation caused by these is compensated by using, in addition to the measuring wavelength, another wavelength at which the oil effects no absorption. The procedure combines the accuracy and reliability of the laboratory method and the advantage of high measuring speed. Its hydraulics part is, however, still complicated, comprising an ultrafilter, a pipe line for the reference sample, pumping means for the reference liquid and a heat exchanger.